There is conventionally known a method of using a radio access network (e.g., UTRA (3G), E-UTRA (LTE), or GERAN (2G)) managed by a telecommunications carrier to provide a so-called extension service which is a communication service between radio communication terminals (sometimes abbreviated as terminals hereinbelow) belonging to a particular entity such as a company (e.g., Non-patent document 1). Such an extension service enables terminals belonging to a particular entity, i.e., an extension group, to communicate with each other via a private branch exchange (PBX) server deployed on a network connected to the radio access network.
Further, in recent years, so-called Device-to-Device (D2D) communication is being discussed by 3GPP. In D2D communication, a radio communication terminal performs direct communication with another communication terminal without involving a radio base station (for example, Non-patent document 2). Such D2D communication does not involve a radio base station or a network apparatus, and therefore can enhance the possibility that the terminal can communicate even in a case where the above private branch exchange server has a failure or congestion.